Elucidating the mechanisms that control morphogenesis is one of the greatest challenges facing modern biology. Vertebrate limb placement is one model system where the mechanisms of morphogenesis can be investigated. The long-term objectives to which the proposed study is subservient are to determine what parameters establish the location and size of a limb on the vertebrate body, and to determine how those parameters misfunction in birth defects involving the limbs. What determines that a certain region of an embryo will develop a limb and that the adjacent tissues will not? Are the limbs unique in possessing limbness qualities? What tissues impart limb qualities to the limb territories and over what time period? It has been demonstrated that stage 11-14 chick flanks can form limbs when grafted. The object of the proposed study is to determine the source of that limb-forming potential. Two alternative hypotheses are advanced: (1) the flank's limb-forming potential is derived from the embryo axis, most likely from Hensen's node, or (2) it is derived from the adjacent limb territories. The distinction between these two alternatives is of fundamental importance to morphogenesis because it is important to know the nature of the influence that Hensen's node exerts over limb placement and development. Is the influence of the node a general signal that is transmitted to the entire lateral plate of the embryo or is it level specific? Determining if the flank's limb-forming ability is derived from Hensen's node will help answer that question. Foil barriers will be implanted into stage 10-11 embryos to isolate the flank from the axis or limbs. At stage 13-14, the flanks will be grafted into the celom of host embryos. If barriers placed between the flank and axis inhibit limb formation in the explanted flank, the concept that the flank's limbness qualities originate in the axis will be supported. If barriers between the flank and the adjacent limbs inhibit limb formation in the explanted flank, the concept that the flank's limbness qualities originate in the adjacent limbs will be supported.